An optical disk as an optical recording medium has almost reached the limit in its resolution of an optical system as a product of Blue-Ray system utilizing a blue-color laser diode and an objective lens corresponding to high NA and is thought to be required to introduce in near future a multilayer structure of recording layer of an optical disk. In such a multilayer optical disk, it is required to detect almost identical light power from each layer and therefore reflectivity of the irradiation light of a particular layer cannot help thinking of being lowered. However, the optical disk is further required to attain high-speed dubbing of video contents in combination with improvement in recording capacity. In addition, since high transfer rate of data is also still improved, it may be assumed, if the present situation is continued, that it will become impossible to acquire sufficient S/N ratio of the reproduction signal. Accordingly, realization of high S/N ratio of a detection signal is essentially required to simultaneously conduct realization in further improvement in multilayer structure and high-speed operation in future.
A technology for high S/N ratio of regeneration signal of an optical disk is disclosed, for example, in the patent document 1 (JP-A No. 1993-342678) and the patent document 2 (JP-A No. 1994-223433) or the like. These patent documents relate to a high S/N ratio of regeneration signal of a magneto-optical disk, aiming at increasing a light power of the light not irradiating an optical disk in order to amplify amplitude of a very weak signal, by branching the light from a laser diode before radiation to an optical disk for multiplexing with a reflected light from the optical click. In differential detection between a transmitted light and a reflected light of a polarization beam splitter that has been used in the related art for signal detection of the magneto-optical disk, detection is substantially conducted through amplification of an orthogonally polarized element with incident polarization with interference of an original incident polarization element and a polarized element orthogonally crossing the incident polarization direction generated with polarization rotation due to the magneto-optical disk. Accordingly, a signal can be amplified by increasing the original incident polarization element, but an incident light intensity of optical disk must be controlled to intensity lower than a certain predetermined level in order to prevent that data is erased or overwritten. Meanwhile, in the related art explained above, a light to be interfered with a signal light is previously separated therefrom, and this signal is interfered with the signal light without condensation to the disk in view of intensifying the light to be interfered for signal amplification without relationship with light intensity at the front surface of disk. Therefore, in the principle, the more intensified light intensity is, the more the S/N ratio can be increased in comparison with noise of an amplifier for voltage conversion of a photo-electric current from an optical detector and shot noise of the optical detector.
In the patent document 1 (JP-A No. 1993-3426778), a couple of lights are interfered with each other to detect interference intensity. In this case, an optical path length of non-reflected light of disk to be interfered is set variably, aiming at acquisition of interference signal amplitude. In the patent document 2 (JP-A No. 1994-223433), differential detection is also conducted in addition to detection of interference intensity. Therefore, high S/N ratio is obtained by cancelling intensity element of each light not making contribution to signal and also cancelling noise element of these lights. In this case, a non-polarizing beam splitter is used for differential detection.